This invention relates to a process of catalytically reducing NO which is contained in an SO.sub.2 -containing gas by a treatment with NH.sub.3 as a reducing agent, wherein the gas that contains SO.sub.2 and NO is mixed with NH.sub.3, the mixture is fed to the catalyst at a rate of 0.2 to 20 standard liters per minute and per gram of catalyst and is reacted there in a fluidized bed under normal pressure at a temperature between 185.degree. and 500.degree. C., and the catalyst has a particle size between 0.1 and 3 mm and is composed of an acid support and an active component, which consists of V.sub.2 O.sub.5, MnO.sub.2, CuO, Fe.sub.2 O.sub.3, NiO and/or Y.sub.2 O.sub.3 and has been applied to the support in a quantity of 0.5 to 20% by weight.
Such process has already been proposed in German Patent Application P No. 36 33 493.6, but it has been found that in that process part of the SO.sub.2 contained in the gas reacts with the catalyst during the NO-reducing reaction. For this reason it is an object of the invention so to improve the previously proposed process that the activity of the catalyst is maintained on a high level for a long time.